dark blue vs navy

dark blue and navy both are nouns.

Word	Noun	Adjective	Verb	Adverb
dark blue	Yes	No	No	No
navy	Yes	No	No	No

As nouns, dark blue and navy are synonyms defined as:

Other synonyms of dark blue include navy blue.

dark blue (noun)	navy (noun)
a dark shade of blue	a dark shade of blue
	an organization of military vessels belonging to a country and available for sea warfare